arXiv:1209.3273v1 [astro-ph.GA] 14 Sep 2012

Astronomy & Astrophysics

manuscript no. tr27˙arxiv

 ESO 2018

May 25, 2018

Photometric distances to young stars in the inner galactic disk

II. The region towards the open cluster Trumpler 27 at L = 355

o⋆

⋆⋆

Gabriel Perren

, Ruben A. V´azquez

, and Giovanni Carraro

3,4

Instituto de F´ısica de Rosario, IFIR (CONICET-UNR), Parque Urquiza, 2000 Rosario, Argentina e-mail:

perren@ifir-conicet.gov.ar

Facultad de Ciencias Astron´omicas y Geof´ısicas (UNLP), Instituto de Astrof´ısica de La Plata (CONICET,UNLP), Paseo del Bosque

s/n, La Plata, Argentina e-mail: rvazquez@fcaglp.unlp.edu.ar

ESO, Alonso de Cordova 3107, Casilla 19100 Santiago de Chile, Chile e-mail: gcarraro@eso.org

Dipartimento di Astronomia, Universita’ di Padova, Vicolo Osservatorio 5, I−35122 Padova, Italy

Received May 15, 2012 / accepted May16, 2012

ABSTRACT

Context.

The spiral structure of the Milky Way inside the solar circle is still poorly known because of the high density of the material

that causes strong extinction towards the galactic center.

Aims.

We present results of the ﬁrst extensive and deep CCD photometric survey carried out in the ﬁeld of the open cluster Trumpler

27, an object immersed in a region of extremely high visual absorption in the constellation of Sagittarius not far from the Galaxy

center. The survey covers almost a quarter of square degree.

Methods.

We look for young stars clumps that might plausibly be associated with spiral structure. Wide-ﬁeld UBVI photometry

combined with infrared information allows us to reconstruct the distribution in reddening and distance of young stars in the ﬁeld

using the Color-Color and Color-Magnitude diagrams

Results.

The analysis of our data, combined too with extensive spectroscopy taken from literature shows that the real entity of

Trumpler 27 as an open cluster is far from being ﬁrmly stated. In fact, instead of ﬁnding a relatively compact group of stars conﬁned

to a small distance range, we found that stars associated to Trumpler 27 are, indeed, a superposition of early type stars seen along

the line of sight extending over several kiloparsecs beyond even the center of the galaxy. We demonstrate that at each distance range

it becomes possible to generate a color-magnitude diagram resembling that of an open cluster. This way, our analysis indicates that

what was considered an open cluster characterized by a signiﬁcant age spread is a stellar continuum that reaches its maximum number

of stars at approximately 3.5 kpc from the Sun, the distance of the Scutum-Crux arm approximately. At the same time, and after

analyzing the way early type stars distribute with distance, we found some of these stellar groups may be linked, within the distance

errors, with other inner spiral arms of our galaxy, including the Near 3 kpc arm at approximately 5 kpc from the Sun. However,

very young stars by themselves do not seem to trace strongly the inner spiral arms since they are distributed evenly across several

kiloparsecs toward the center of the Galaxy. This is a remarkable diﬀerence with current HI and CO radio observations maps that

show inner spiral arms composed by discrete structures of gas with a well deﬁned inter-arm separation.

Key words. Galaxy: disk – Open clusters and association: general– Open clusters and association: individual: Trumpler 27 - -Stars:

early type – Galaxy: structure

1. Introduction

One of the authors (Carraro 2011) has very recently examined

the parameters of the young diﬀuse stellar populations in the

direction l = 314

◦

as seen in the background of some open

clusters and related them successfully with the inner galactic

structure. This study made use of photometric techniques that

we developed in the last decade and successfully applied to the

third quadrant of the Milky Way (Carraro et al. 2005; Moitinho

et al. 2006; V´azquez et al 2008).

In Carraro (2011), hereafter the ﬁrst paper of the series, we

extensively described the project and the employed methods and

proceeded to study the spatial distribution of early type stars

⋆

Based on observations collected at Las Campanas Observatory,

Chile

⋆⋆

Full photometric data and Tables 2 and 3 are available at the

CDS via anomymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via

http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/999/A999

(spectral types O and B) in the direction of l = 314

◦

. Apart from

the prominent Carina-Sagittarius spiral arm, we detected for the

ﬁrst time signatures of the Scutum-Crux arm in that direction.

These ﬁndings conﬁrmed the predictions of earlier studies on

the location and pitch angles of part of the spiral arm pattern in

the fourth Galactic quadrant.

The present article, the second of the series, is aimed at improv-

ing our knowledge of the spiral structure in the inner galactic

disk in another direction, much closer to the Galaxy center, at

l = 355

◦

. In this case, instead of a single pointing as in the ﬁrst

paper, we combined together three pointings to cover a wide

region (∼ 25 arcmin on a side) in the direction of Trumpler

27. This is an open cluster located in the Sagittarius constel-

lation, 5 degrees from the center of the Galaxy and slightly

below the galactic plane, at coordinates α

2000

= 17h36m20s,

2000

= −33

◦

′

(l = 355

◦

.064, b = −0

◦

.742). The zone

occupied by the cluster is heavily obscured by dust clouds but

nevertheless it is still possible to see a signiﬁcant but sparse

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

group of relatively bright stars. The ﬁrst extensive photometric

survey on this object was conducted by The and Stokes (1970)

who obtained UBV photographic photometry for about 40 stars

spread over a circular area of 11

′

radius around the assumed

center of this cluster. They found huge values for the color

excess E

(B−V)

ranging from -approximately- 0.6 to 2.5 mag.

Despite this, they found that the extinction law is normal with

B−V

= 3.1 and placed the cluster at a heliocentric distance

of 1.1 kpc. Later on, Moﬀat et al. (1977, hereinafter MFJ77)

conducted a deeper UBV photometric survey including pho-

toelectric observations of the brightest stars and photographic

ones for the rest of them. This study was combined with

spectroscopic analysis for a signiﬁcant number of stars (about

50) and suggested that Trumpler 27 is at a distance of 2.1 ± 0.2

kpc from the Sun with almost similar reddening values to the

ones previously computed by The and Stokes (1970). Bakker

and The (1983), used Walraven photometric data combined with

infrared and UVB photometry. This last study conﬁrmed that

the extinction law is normal, i.e. R

= 3.1 − 3.2, that the cluster

is located at a distance of 1.7 ± 0.25 kpc and is about 10

years

old.

After a 20 years hiatus with no investigations, Trumpler 27

was the target of a new spectroscopic campaign carried out by

Massey et al (2001, hereinafter MDEW01) where the oldest

spectral types were veriﬁed and spectroscopy was made for

several stars that had not previously been measured. This new

spectroscopic survey revealed a surprisingly hot and massive

stellar population in the area of this object: one M supergiant,

10 supergiants of B-types, two Wolf-Rayet stars, one Of-type

star, two O-type giant stars, other stars of B-types and G0I

-probably- Cepheid star. In MDEW01 the parameters of several

stars in Trumpler 27 were redetermined onto the basis of this

new spectral survey but retaining the old UBV photometry

made by MFJ77. In this investigation the authors found that

the cluster is located at a distance larger than any previous

determination: 2.5 kpc from the Sun. The next interesting

ﬁnding made by MDEW01 is the detection of a signiﬁcant

age spread amongst the B-type stars. It appears that some of

them show ages ranging from 6 to 10 × 10

years being about

2 to 4 × 10

older than the rest of them. They assume this

age spread comes from the large diﬃculty in separating clus-

ter members from backgroundstars in this highly obscured zone.

Given its position -close to the Galaxy center and onto the galac-

tic plane- Trumpler 27 is an ideal target for the purpose of inves-

tigating part of the spiral structure in the fourth Galactic quad-

rant. Its surroundings contain a large number of potential blue

stars, some of which are considered excellent tracers of spiral

arms such as W-R stars and O-type stars as shown by MDEW01

and MFJ77. It is useful to recall that it has been almost 30 years

since the last photometric study on this object and that we are

now in the position of taking advantage of the large number of

spectral types to re-discuss the distance and age of Trumpler 27

with modern UBVI observations. These two parameters are of

paramount importance because this cluster may become a good

galactic spiral arm tracer if the distance discrepancy of the order

of about 1.4 kpc from one author to other is removed. In other

words, such a diﬀerencemeans that the cluster maybelong to the

Carina-Sagittarius arm or the Scutum-Centaurus arm. But, ﬁrst,

we should assess whether Trumpler 27 really exists and if the

found distance discrepancies are only due to the treatment of the

strong reddening of the region and/or to the fact that in clusters

so heavily obscured, like this one, only the upper -vertical- main

sequence is useful for ﬁtting a reference line (e.g., the ZAMS of

Scmidt-Kaler (1982)).

In Section 2 we discuss the data acquisition, the reduction

process, the stellar astrometry and the comparison with former

photometry in the region. In Section 3 we show and discuss the

main photometric diagrams and introduce the strategy to derive

intrinsic parameters of stars in Trumpler 27. In Section 4, we an-

alyze the data set derived for hot stars and the natureof Trumpler

27. Section 5 includes a description of ﬁndings using infrared

data. In Section 6 we collect the evidence to interpret the struc-

ture of the inner Galaxy as seen through the Trumpler 27 region.

Conclusions are given in Section 7.

2. Observation and data reduction

Three regions containing Trumpler 27 were observed in the

UBVI photometric system at Las Campanas Observatory (LCO)

on four diﬀerent runs in 2006, 2010, and 2011, as illustrated in

Table 1, which lists useful details of the observations, like ﬁlter

coverage, airmass range and exposure time and sequences. We

used the SITe#3 CCD detector onboard the Swope 1.0m tele-

scope

. With a pixel scale of 0.435 arcsec/pixel, this CCD al-

lowed us to cover 14.8 × 22.8 arcmin on sky. In all runs the

seeing was good, ranging from 0.9 arcsec to 1.8. However, post-

processing indicated that in the 2006 and 2010 runs, the nights

were not completely photometric, and for this reason we only

rely on the 2011 standard stars. The whole ﬁeld of view is shown

in Fig 1, a montage of all V exposure images, and covers an area

of 26 × 23 squared arcmin. North is up, and East to the left.

To determine the transformation from our instrumental system

to the standard Johnson-Kron-Cousinssystem, and to correct for

extinction, we observed stars in Landolt’s areas PG 1323, PG

1633, and MarkA (Landolt 1992) multiple times and with diﬀer-

ent air-masses ranging from ∼ 1.07 to ∼ 2.0, and covering quite

a large color range -0.3 ≤ (B − V) ≤ 1.7 mag. All the images

from previous runs have been then shifted to the 2011 run by

means of all common stars, before transforming the photometry

into the standard system.

2.1. Basic photometric reduction

Basic calibration of the CCD frames was done using IRAF

package CCDRED. For this purpose, zero exposure frames

and twilight sky ﬂats were taken every night. All frames were

pre-reduced applying trimming, bias and ﬂat-ﬁeld correction.

Before ﬂat-ﬁelding, all frames were corrected for linearity,

following the recipe discussed in Hamuy et al. (2006).

Photometry was then performed using the IRAF

DAOPHOT/ALLSTAR and PHOTCAL packages. Instrumental

magnitudes were extracted following the point-spread function

(PSF) method (Stetson 1987). A quadratic, spatially variable,

master PSF (PENNY function) was adopted, because of the

large ﬁeld of view of the detector. Aperture corrections were

then determined making aperture photometry of a suitable

number (typically 15 to 20) of bright, isolated, stars in the

ﬁeld. These corrections were found to vary from 0.160 to 0.290

http://www.lco.cl/telescopes-information/henrietta-swope/

IRAF is distributed by the National Optical Astronomy

Observatory, which is operated by the Association of Universities

for Research in Astronomy, Inc., under cooperative agreement with the

National Science Foundation.

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

Table 1. UBVI photometric observations of Trumpler 27 and standard stars.

Target RA DEC Date Filter Exposure (sec) airmass (X)

Field-1 17:36:20.79 -33:28:02.8 2006 May 31 U 20, 200, 2x1500 1.02−1.05

B 10, 100, 2x1200 1.00−1.00

V 5, 10, 100, 900 1.14−1.16

5, 10, 2x100 1.08−1.10

Field-1 17:36:20.79 -33:28:02.8 2006 Jun 26 B 40 1.00−1.01

V 20 1.00−1.01

20 1.00−1.01

Field-1 17:36:20.79 -33:28:02.8 2006 Jun 31 U 60 1.00−1.01

B 30 1.00−1.01

V 30 1.00−1.01

30 1.00−1.01

Field-1 17:36:20.79 -33:28:02.8 2010 May 08 U 15, 90, 300, 1500 1.02−1.03

B 5, 45, 300, 1200 1.06−1.10

V 2x5, 120, 900 1.10−1.15

2x5, 30, 600 1.01−1.01

Field-2 17:36:54.79 -33:28:02.8 2010 May 08 U 15, 90, 300, 1500 1.01−1.01

B 5, 45, 300, 1200 1.03−1.04

V 2x5, 120, 900 1.12−1.14

2x5, 30, 600 1.05−1.08

Field-3 17:35:47.79 -33:28:02.8 2010 May 08 U 15, 90, 300, 1500 1.05−1.11

B 5, 45, 300, 1200 1.11−1.18

V 2x5, 120, 900 1.36−1.42

2x5, 30, 600 1.20−1.28

Field-1 17:36:20.79 -33:28:02.8 2011 Jun 03 U 60, 180 1.00−1.01

B 30, 120 1.00−1.01

V 10, 60 1.00−1.01

10, 60 1.00−1.01

Mark A 20:43:55.09 -10:45:38.0 2011 Jun 03 U 180, 2x240 1.07−1.60

B 120, 2x180 1.07−1.56

V 60, 2x90 1.07−1.53

60, 2x90 1.06−1.51

PG 1323 13:25:38.91 -08:50:05.6 2011 Jun 03 U 3x180 1.08−1.73

B 3x120 1.08−1.77

V 3x60 1.07−1.82

3x60 1.07−1.85

PG 1633 16:35:24.91 09:47:39.8 2011 Jun 03 U 2x180, 240 1.29−1.83

B 2x120, 180 1.29−1.88

V 3x60, 90 1.29−2.02

2x60, 90 1.29−1.97

mag, depending on the ﬁlter. The PSF photometry was ﬁnally

aperture corrected, ﬁlter by ﬁlter.

2.2. Photometric calibration

After removing problematic stars, and stars having only a few

observations in Landolt’s (1992) catalog, our photometric solu-

tion for the run was extracted combining measures from both

nights- after checking that they were stable and similar- yielding

a grand total of 63 measurements per ﬁlter, and turned out to be:

U = u+ (5.004±0.010)+ (0.49±0.010)×X+ (0.129±0.013)×

(U − B)

B = b+ (3.283 ±0.006)+ (0.25 ±0.010)× X+ (0.040 ±0.008)×

(B − V)

V = v+ (3.204±0.005) + (0. 16±0.010)×X−(0.066±0.008) ×

(B − V)

I = i+(3.508±0.005)+(0.08±0.010)×X+(0.037±0.006)×(V−I)

where X indicates the airmass.

The ﬁnal r.m.s of the ﬁtting in this case was 0.040, 0.019, 0.015,

and 0.015 in U, B, V and I, respectively.

Global photometric errors were estimated using the scheme de-

veloped by Patat & Carraro (2001, Appendix A1), which takes

into account the errors resulting from the PSF ﬁtting procedure

(i.e., from ALLSTAR), and the calibration errors (corresponding

to the zero point, color terms, and extinction errors). In Fig. 2 we

present our global photometric errors in V, (B−V), (U −B), and

(V − I) plotted as a function of V magnitude. Quick inspection

shows that stars brighter than V ≈ 20 mag have errors lower than

∼ 0.05 mag in magnitude and lower than ∼ 0.10 mag in (B − V)

and (V − I). Higher errors, as expected, are seen in (U − B). The

ﬁnal catalogue contains 9769 UBVI entries.

2.3. Astrometry

The optical catalogue was cross-correlated with 2MASS, which

resulted in a ﬁnal catalog including UBVI and JHK

magni-

tudes. As a by-product, pixel (i.e., detector) coordinates were

converted to RA and DEC for J2000.0 equinox, thus providing

2MASS-based astrometry, useful for e.g. spectroscopic follow-

up.The rms of the residuals in the positions were 0.

′′

15, which is

about the astrometric precision of the 2MASS catalogue (0.

′′

12,

Skrutskie et al. 2006 ).

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

δ (2000)

α (2000)

-33.65

-33.60

-33.55

-33.50

-33.45

-33.40

-33.35

-33.30

17.59017.59517.60017.60517.61017.61517.62017.625

Fig.1. Montage of all V images (28) taken in the area of

Trumpler 27. The ﬁeld is about 26 arcmin × 23 arcmin. α and

δ coordinates are given in decimal notation. North is up, East to

the left.

Fig.2. Trend of global photometric errors in magnitude and col-

ors as a function of our V magnitude. See text for details.

2.4. Comparison with former photometry

Up to now, the most extensive catalogue of UBV photometry

in the region of Trumpler 27 belongs to MFJ77. The other pho-

tometric sets are only photographic or in the Walraven system.

Comparing our data set with MFJ77 we found 83 common stars

in V, 81 in (B−V) and 68 in (U−B). The diﬀerences in the sense

of our data minus MFJ77, are shown in Fig. 3 as a function of

our V. Most of the stars in the MFJ77 survey have photographic

photometry and just a few have photoelectric photometry. In the

comparisonprocess, we found at least half a dozenstars showing

strong diﬀerences in apparent magnitude and/or color indices.

Fig.3. Diﬀerences of our photometry with MFJ77 plotted

against our V magnitudes. Upper panel, δ

U−B

, mid panel δ

B−V

lower panel δ

Fig.4. The CCD -upper left panel- and the CMDs for (B − V)

-upper right panel-, for (U − B) lower left panel and for (V − I)

-lower right panel of all observed stars. Solid line in the CCD

is the Schmidt-Kaler (1982) intrinsic line. Dashed lines in the

CCD mean the path of the reddening for a B5- and an A0-type

stars as indicated by the labels. The bluish region represents the

location of nearby dwarf stars; the yellowish one points out the

locus occupied by A-F-type stars aﬀected by increasing redden-

ing. Stars above the B5 reddening line are young stars (see text),

except those ones for which no reddening solution is possible.

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

This may be produced, specially in crowded regions, in an un-

avoidable star-lightcontaminationby nearby neighborswhen us-

ing diaphragms in photoelectric measures. Mean diﬀerences and

standard deviations are

= −0.08±0.15,δ

(B−V)

= −0.19±0.15

and

(U−B)

= −0.23 ± 0.17. The systematic trend in Fig. 3, in

the sense that our values are brighter and bluer than those of

MFJ77 and the large scatter, is likely produced by some zero

point diﬀerences between our photometry and MFJ77’s photo-

graphic and photoelectric photometry made with diﬀerent tele-

scopes and diﬀerent diaphragm apertures, respectively.

3. The all area photometric diagrams

Earlier CCDs and CMDs of the region of Trumpler 27 have been

always very diﬃcult to interpret because of strong and variable

internal reddening and the consequent diﬃculties in deﬁning the

upper main sequence. Besides, the upper part of Trumpler 27

CMD is quite vertical and short what makes any ﬁtting of the

ZAMS (Schmidt-Kaler 1982) dubious. The sum of these diﬃ-

culties have precluded this object from a more in-depth study so

far (McSwain & Giess, 2005).

Figure 4 shows the CCD (upper left panel), the V vs (B −

V) (upper right panel), the V vs (U − B) (lower left panel) and

the V vs (V − I) (lower right panel) CMDs. The solid line in

the CCD is the ZAMS of Schmidt-Kaler (1982) shown in the

normal R

= 3.1 position. The reddening path for a B5-type star

is also indicated. A simple visual inspection of the CCD allows

to establish three dominant stellar populations:

• a ﬁrst one, above the reddening line for a typical B5-type

star, composed undoubtedly of blue stars strongly aﬀected

by variable reddening;

• a second stellar population appears below the reference line

for a B5 star (the yellowish zone) composing a sort of stellar

band starting at the second knee of the ZAMS and extending

to the red side because of diﬀerent amounts of reddening af-

fecting them. This large star concentration corresponds,pho-

tometrically speaking, to A and early F-type stars but also to

reddened B-type stars later than B5 as indicated by the path

of the reddening in the CCD;

• ﬁnally, a third stellar population (greenish coloured) little af-

fected by reddening appears extending from the domain of

A to M stars all along the intrinsic reference line. Most of

these stars are likely to be nearby objects since visual ab-

sorption maps from Neckel and Klare (1980) reveal a sud-

den absorption increment at less than 1 kpc in this direction.

Therefore, the detection of intrinsically faint stars not red-

dened or just marginally aﬀected by interstellar dust is only

possible if they are close enough to the Sun.

Inspection of Fig. 1 lends further support to this scenario:

intense absorptionis seen at the west-southwest andimmediately

north of the center of Trumpler 27, and also to the east. A clear

star overdensity is seen, in the same ﬁgure, at the northeast and

at the expected location of the cluster core.

The CMDs [V vs (B−V), V vs (U − B) and V vs (V − I)] are

less illustrative since not much can be extracted from them be-

yond the presence, aboveV = 16 mag, of an extended upper stel-

lar sequence with a high degree of color dispersion that reﬂects

the strong inﬂuence of dust in the zone. Since the three popula-

tions discussed previously are undistinguishable from each other

in the CMDs, we estimate this type of diagram plays a marginal

role to inferring useful information on this region.

3.1. Deriving the parameters of early-type stars

If the CMD is segmented with magnitude intervals and the cor-

responding CCD to each interval is built, it becomes simpler

to understand the way stellar populations develop in a given

zone. The foundation and applications of this kind of analysis

in open clusters and Galactic ﬁelds is developed in Carraro et

al. (2008, 2009). In the present case the procedure is shown

in the series of CCD in Fig. 5 at intervals of 1 mag down to

V = 18. The superimposed lines follow the slope E

(U−B)

0.72×E

(B−V)

+ 0.05×E

(B−V)

explained long time ago by Hiltner

and Johnson (1956) (correspondingto R

= 3.1) for stars of var-

ious spectral types from O to B5 in addition to the A-type stars.

The series of panels shows the color range covered by the

three above mentioned star populations. Actually, reddened hot

stars along the reddening lines for O and B4 types look well

detached from the rest down to V ∼ 14. A comparison of our

CCD with the ones produced by Bakker and The (1983), in their

Fig. 3, or in Fig. 5 of MFJ77, suggests that the major diﬀerence,

apart from the inclusion of many faint stars, rests in the high

number of newhot and reddenedstars detected in our ﬁeldwhich

in turn is larger than the others.

The panel in the right lower corner shows the overall CCD

where we have identiﬁed, with open squares, bright stars already

studied in previous articles via spectral analysis.

From V ∼ 14 a stellar group (yellowish coloured) composed

of a wide strip of A and F-type stars becomes evident reaching

the maximum at V = 15. Certainly, this latter group shows an

ample range of reddening. Beyond this limit dwarf stars of G-K-

and even some M-type stars are detected at increasing number all

along the intrinsic line indicating they are not so much aﬀected

by reddening and belong to the greenish zone. As stated above,

these stars and part of the less reddened A-F- stars in the group

belong to a nearby population in the line of sight to Trumpler 27.

In the remaining of this paper we shall focus on those stars

above the reddening line for B5-type stars as indicated in Fig.

5 to derive individual distances by means of the spectroscopic

parallax method since we know they have unambiguous red-

dening solution in the CCD. This method has proved to be

very eﬃcient to approximate spectral types from photometry

alone when unique reddening solutions are available as shown

in Ahumada et al. (2011). Intrinsic colors for each star in this

zone of the CCD were derived using the reddening line slope

(U−B)

= 0.72 × E

(B−V)

+ 0.05 × E

(B−V)

along with the relation

(U − B)

= 3.69 × (B − V)

+ 0.02 (Carraro et al. 2010). This

gives a second order equation where the (B − V)

index is the

positive root and the (U − B)

index comes from the above ex-

pression. This locates the star on a point in the ZAMS (Schmidt-

Kaler 1982) where the nearest spectral type is assigned. Unlike

the Q-method that derives the (B − V)

star color assuming a

linear relation of color excesses, here we adopt a quadratic rela-

tion that provides a more accurate solution when the color excess

is high. Once individual color excesses and intrinsic colors are

derived,we proceed to assign spectral types and absolute magni-

tudes following the relationship given by Schmidt-Kaler (1982).

In this procedure all stars are assumed to be of luminosity class

V. However, a good number of hot stars have spectral types and

luminosity class from MDEW01 and MFJ77. In these cases, the

assignment of intrinsic color and absolute magnitude depend on

the spectral type and luminosity class found by these authors us-

ing again the Schmidt-Kaler (1982) calibrations. Individual dis-

tance moduli are given after correcting apparent magnitudes by

= 3.1×E

(B−V)

. We accept R = 3.1,which is completelyin line

with the rigorous study of the absorption throughout Walraven

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

Fig.5. The CCDs of the segmented CMD at intervalsof 1 mag. Dashed line shows the position of the Schmidt-Kaler (1982) intrinsic

line. The succession of solid lines show the path of the reddening for some of the early type stars indicated with the respective labels.

The panel in the right lower corner shows the overall stars and those with spectral types, grey squares, from MFJ77

Table 2. Intrinsic photometric values, distances and spectral types of stars in Trumpler 27

Id E

(B−V)

(B − V)

(U − B)

d σ

Id(MFJ77) ST

(MFJ77,MDEW01)

5517 1.69 -0.28 -1.00 -2.60 3.17 0.73 B0.5 (...) (...)

4732 1.10 -0.28 -1.00 -2.60 2.64 0.58 B0.5 19 OB-

5355 1.14 -0.21 -1.02 -6.90 6.60 0.29 B0.5Ia 16 B0.5Ia

6456 1.12 -0.02 -0.62 -7.10 6.74 0.15 B0Ia 2 B0Ia

5672 1.80 -0.27 -0.95 -2.35 3.38 0.78 B1 (...) (...)

Id ﬁrst column is our star numbering. Id column nine is the MFJ77 numbering. ST

indicates the photometric spectral type

found in this paper. ST

(MFJ77,MDEW01) indicates spectral types from the literature.

Table 3. Idem for stars outside Trumpler 27

Id E

(B−V)

(B − V)

(U − B)

d σ

Id(MFJ77) ST

(MFJ77,MDEW01)

442 1.22 -0.25 -0.90 -2.10 3.41 0.84 B1.5 (...) (...)

4497 1.09 -0.25 -0.90 -2.80 2.83 1.33 B1.5:V: 32 B1.5:V:

3629 1.15 -0.26 -0.95 -3.20 3.22 0.95 B1:V: 34 B1:V:

8932 1.25 -0.26 -1.00 -5.40 2.64 0.40 B1II 103 B1II

Columns as in Table 2.

and infrared photometry developed by Bakker and The (1983)

in the region.

The distances obtained are photometric and are subject,

therefore, to uncertainties depending primarily on photometric

errors. As donein Carraro (2011),we analyze the impact of these

errors on distances by starting with the well known expression

for the distance modulus:

V − M

= −5 + 5 × log(d) + A

(1)

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

Fig.6. The left panel is the contour plot showing the star density in the surveyed area generated with 9000 stars approximately.

The insert gives the number of stars per half-minute squared box. The right panel shows the contour plot of those stars early than

B5-type (see text). The insert as in the left panel. The 600 pxl circle indicates the region assumed to be the core of the cluster.

Fig.7. The left panel stands for the spatial distribution of early type stars according to their spectral types -shown in the insert- as

explained in the text. The right panel is the contour plot of E

(B−V)

across the cluster surface. The insert gives the amount of E

(B−V)

The circle has the same meaning as in Fig. 6.

A simple error propagation leads to:

(d)

= ln(10) × d × 0.2 ×[σ

+ 3.1 × σ

(B−V)

] (2)

Assuming that ǫ

)

= 3.1×ǫ

(B−V)

, where σ

(V)

and σ

(B−V)

directly

come from photometry. In the ﬁnal error computation we adopt

)

= 0.

The results of this procedure yielded about 600 stars with

intrinsic parameters and distances as indicated in Tables 2 and

3 in a self explanatory format. Inspecting the errors in distance

in the same tables, it follows that typical errors are about 20%,

quite reasonable for the method and of the same order of the

obtained by the usual ZAMS (e.g. Schmidt-Kaler 1982) super-

position. We want to emphasize that the assignment of spectral

types entirely rests on the star positions onto the CCD (Carraro

2011) as if they were single stars. We recall that some of them

may be undetected binaries or unresolved double stars what in-

troduces another source of distance uncertainty due to altered

colors and magnitudes. Even if we deal with true single stars

having MK spectral types this is not enough to reduce uncer-

tainties in distances since the MK classiﬁcation and therefore

the assigned M

may vary from author to author (see cases of

diﬀerent classiﬁcation in Tables 2 and 3). Finally, it is evident

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

that even amongst the stars located along the reddening line for

B5-type stars there is still a chance of contamination by intrinsic

data scatter of some reddened A-type stars. Therefore, and in an

attempt to minimize wrong star inclusions, we restrict the spec-

troscopic parallax method to those stars in the range from B4.5

to O-types (unless otherwise stated). We are conﬁdent that, de-

spite the errors, our large data sample will clearly revealthe main

propertiesof the stellarpopulations in the region of Trumpler 27.

3.2. Discussion on spectral types and distances of some

stars

Here we brieﬂy discuss the nature and parameters of a few

particular stars listed in Tables 2 and 3 for which distances

have been derived using the spectroscopic parallax method.

We focus mainly on some stars for which MK spectral types

and thin-prism classiﬁcation comes from the literature. We also

discuss brieﬂy some potential variable stars. We refer to all

these stars using the identiﬁcation given in MFJ77.

Star 1: MDEW01 conﬁrm the oldest spectral type, M0Ia,

given by MFJ77. It is also a suspected variable (NSV 22849)

as indicated by Samus et al. (2010). The distance of 1.8 kpc

has been derived under the assumption that it is not a variable.

Additional information on this objects is commented later when

using infrared data.

Star 2 (LSS 4253): the old spectral type from MFJ77,

O9Ia, turns out to be B0Ia according to MDEW01 with no

indication about membership. This last spectral type and our

new photometry locate it at almost 7 kpc from the Sun, outside

any reasonable distance limit given for Trumpler 27 in the

literature.

Stars 8, 10, 12, 19, 20, 21, 25 and 30, all have thin-prism plates

classiﬁcation in MFJ77 (a classiﬁcation technique that indicates

whether a star is a potential OB star) and none of them were

studied spectroscopically by MDEW01. In most of these cases

they were classiﬁed as potential OB stars with diﬀerent degrees

of accuracy as seen in Tables 2 and 3. Our procedure conﬁrms

the MFJ77 thin-prism plates classiﬁcation approximation as

they resulted of photometric spectral types O4, O7, O8.5, B0.5,

B1.5, B1, O4 and O6-types, respectively.

Star 16 (LSS 4263): the early spectroscopy gave O9.5 II: but

changed to B0.5 Ia after MDEW01. The new spectral type and

our photometry locate it at a distance of about 7 kpc.

Star 23: The spectral type determined by MFJ77, B0.5Ia, was

changed to B0.7Ia after MDEW01. This star is also known as

V1082 (Samus et al. 2010), a probable eclipsing variable. We

derived a distance of 3.5 kpc but if it becomes a true binary star,

the distance could be reduced to 2.6 kpc in the most favorable

case of similar masses.

Star 44: this star is a B1.5Ia according to MDEW01 who

have given concluding spectroscopic arguments favoring it is a

supergiant star and not a giant one as stated in MFJ77. In view

of the distance of 2.5 kpc they computed for Trumpler 27, they

assumed this star is not a cluster member, an assertion we agree

with because the distance we obtain with our photometry is

about 16 kpc from the Sun. Even if we assume it is a binary star

its distance would still be more than 10 kpc away from the Sun.

The WR stars: on one side, star 28 (WR 95) was classiﬁed

WN5 by MFJ77 and WC9 by MDEW01; we adopted here this

latest and modern classiﬁcation. On the other side, star 105

(HDE 318016) is a WC7/ WN6 following MDEW01. Conti and

Vacca (1990) reported distances of 2.8 kpc for the WC9 and 2.4

kpc for the WC7/WN6 that were adopted by MDEW01 in their

analysis. To deal with these two complex stars we proceeded to

derive their intrinsic (B −V) colors by means of the relationship

between the Smith (1968) ubv and UBV systems as given in

Lundstr¨om & Stenholm (1984). According to the relationship

given in this last paper, it was assumed (b − v) = −0.4 for the

WC9 (WR 95) and (b−v) = −0.3 for the WC7/WN6 (105) while

the absolute magnitudes were taken from the newest values

given by van der Hucht (2001). Excesses of color obtained using

the relationships of Smith (1968) were E

(B−V)

= 1.95 for WR

95 and 1.4 for star 105. Finally we derived distances of 2.2 for

WR95 and 3.5 kpc for star 105 that we will use in the analysis

next section.

Star 107 (LSS 4257): it was classiﬁed B0V by MFJ77 but the

new spectral type from MDEW01 drastically changes the point

by setting this star as a supergiant of a later type, B0.5Ia. As

in the previous case of star 44, they concluded this is another

non-member star. We agree since our distance sets this star at

more than 10 kpc from the Sun. Even in case of binarity, its

distance would be reduced to 7.6 kpc, too far from any previous

estimate of the cluster distance.

Star 102 (V925 Sco, HD159378): this is a variable star with

no well established period from 70

to more than 300

and

light variation of a few tenths of a magnitude (Samus et al.

2010). Since, according to van Genderen (1980), it may not be a

Cepheid variable of Pop I, we treated it as a G0 supergiant star

and derived a distance of 2.4 kpc not far from the 2.1 kpc given

by MFJ77. Parthasarathy & Reddy (1993) placed this object at 3

kpc from the Sun but they gave no precise indication on the way

they got their reported distance.

4. Hunting for Trumpler 27

We made an attempt to use proper motions in the area of

Trumpler 27 from the UCAC3 data-base (Zacharias et al. 2010).

The putative distance (at 2-3 kpc from the Sun) and the crowd-

ing of the region towards the Galactic center made the exercise

useless. Errors in both proper motion components are large and

no feature can be detected. This is exactly as in Moni Bidin et al.

(2011), where three possibly new globular clusters are discussed

toward the Galactic bulge using data from the VISTA VVV sur-

vey (Minniti et al. 2011). As in that case, proper motions are so

scattered that we even restrain from showing the results.

In previous papers we have explained (see e.g. V´azquez et al.

2010) that instead of using star density proﬁles to set the cluster

boundaries we prefer to look for the zone of higher star density

in a contour plot. Indeed, density proﬁles reduce an open cluster

-irregular in shape by deﬁnition- to a centrally peaked spherical

stellar density distribution. On the contrary contour plots reveal

details of the impact of absorption on the cluster shape and allow

to deﬁne the boundaries in a more realistic approach. The pro-

cedure works well in areas where the absorption is high like the

one in this paper. As in the case of a radial density proﬁle we set

the size of a cluster to the area enclosed when the contour plot

reaches a ﬂat value. Notwithstanding and for the sake of com-

pleteness of the present analysis, in the next section we shall ex-

amine the infrared density proﬁle. Meanwhile, Fig. 6 (left panel)

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

shows the contour plot (0.5 × 0.5 arcmin box using the Kriging

gridding method) generated with more than 9000 stars detected

in the region of Trumpler 27. The full contour plot shows no

star overdensity at all in the place where the cluster is assumed

to be (the bright stars near the center of the mosaic in Fig. 1).

Overdensities are detected north and south of the location of the

cluster center conﬁrming what a simple eye-inspection shows in

Fig. 1. Actually, the lack of a notorious overdensity right there

suggests that Trumpler 27 has been detected for being handful

of bright stars well detached from the rest by dust clouds. This is

better shown in the right panel of Fig. 6 which displays the con-

tour map generated only with all early-type stars of Trumpler

27 included in Tables 2 and 3 (spectral types down to B4.5). The

circle in this panel has 600 pxl radio, just over 4 arcmins and sur-

rounds entirely the weak overdensity emerging at the position of

the cluster. That is, the contour map in the right panel shows that

the cluster is just a handful of early stars separated by a thick

dust ring from other similar groups in the region. Figure 7, left

panel, displays stars in Tables 2 and 3 according to their spec-

tral types, describes clearly the above assertion. Note that the

stellar density in this region cannot be assumed diﬀerent from

that north-west and west. The role played by the absorption is

also well depicted by the contour map of E

(B−V)

excesses built

in the right panel of Fig. 7 with the values listed in the tables.

Here we see that reddening varies in the range from 1 to more

than 2 mag across the surface with higher values in a wide band

from the south- south-west to the north-east that separates the

diverse groups seen in the left panel. A comparison of the right

panel in Fig. 7 and the left panel (all stars) in Fig. 6 suggests that

a vast fraction of the A-type stars surveyed in the area may be

immersed in the dust and just a few of them together with oth-

ers of later types are likely located in front of the dust clouds.

Otherwise, the overdensities seen north and south-east in the left

upper panel should disappear in this plot.

The zone of Trumpler 27 was included by Bitran et al. (1997)

in a CO survey focused in a dozen degrees around the Galaxy

center. The cluster is not only immersed in a region of large

CO emission but, most interesting, it is projected against a zone

of extremely high absorption as shown in map 222 made by

Neckel and Klare (1980). This alone justiﬁes the appearance of

our CCDs in Fig 5 where potential A-F-type stars start with lit-

tle absorption to end up, at V = 16, completely immersed in the

dust clouds. Likewise, the latest spectral type stars in the ﬁeld

have to be necessarily nearby stars since they are intrinsically

too faint and do not appear much aﬀected by reddening.

Figure 8 traces the path of reddening with distance inside

and outside the cluster boundaries. Error bars coming from Eq.

(2) are also shown. It is worth mentioning brieﬂy the impact

of distance errors into our analysis. We discussed in previous

section the meaning of distance errors which are mainly based

on the propagation of photometric errors which are random in

nature. Fig. 8 shows the trend of our sample of stars toward

the Galaxy center and we do not see that errors can modify

any of our conclusions. When looking through the core of the

Trumpler 27 region the bulk of early type stars extends for more

than 4 kpc while some others may reach 9 kpc and even more

than 15 kpc as indicated. Something similar happens outside

the 600 pixels limit with some stars located at large distances

too. The reddening behavior shown in this ﬁgure is well in line

with the above mentioned absorption map 222 from Neckel and

Klare (1980). Map 222 and nearby ones show that absorption

increases after 0.5 kpc from the Sun, rises up to 5-6 mag in

the next kiloparsec and keeps high up to 4-5 kpcs towards the

Galaxy center. Our deeper and extended sample of young stars

Fig.8. The path of reddening with distance for stars inside (up-

per panel) and outside (lower panel) 600 pxl radius. Horizontal

bars are distance errors computed from (2). Some of the distant

stars are indicated. Numbers in parentheses mean the MFJ77 no-

tation.

not only conﬁrms ﬁndings of Neckel and Klare (1980) but also

states strong absorption variability all across the region with

0.65 < E

(B−V)

< 2.2 mag meaning more than 6 mag of visual

absorption. Fig. 8 shows too that dust clouds are not a local phe-

nomenon but they are presents for several kiloparsecs starting

near the Sun and spanning over 4 kpc (at least) towards the cen-

ter of the Galaxy.

Histograms for diﬀerent spectral types as a function of dis-

tance for both regions are shown in Fig. 9. Left and right panels

are for stars inside and outside Trumpler 27, respectively. The

upper panels include O-. WR-, B0- and late type supergiants.

Mid panels is for B1-B2- type stars and lower panels is for B3-

B4- types. For the sake ofan easy visualization, we restricted the

plot to all the stars up to 7 kpc from the Sun.

In terms of conﬁrmed spectral types, we found that 19 stars

out of 126 inside the cluster limits have some type of spec-

tral classiﬁcation (MK or objective prism) from Massey and/or

MFJ77 including O- and B-type stars and one WC9 star; the

supergiant star No 1 belongs to this zone. Outside the cluster ra-

dius there are 501 early type stars; 9 of them, including the G0

Cepheid variable, have spectral analysis: one WC7/WN6, one

O8.5III and one early B-type supergiant.

Inside the cluster limits O-type stars are seen nearly at all

distances. The rest of stars show a narrower distribution beyond

1.5 kpc from the Sun. Late dwarf B-types are lacking in the ﬁrst

1-1.5 kpc probably because of dust but also because of an statis-

tical artifact. The lack of them at larger than 4-5 kpc is mainly

due to a combinationof extinctionand intrinsic faintness of these

spectral types. In the left panels there seems to be two peaks, one

at ∼1-1.5 kpc and the other -higher- at ∼3.5 kpc. The 1-1.5 kpc

peak is not far from the estimated distances by The and Stokes

(1970) and Bakker & The (1983) for Trumpler 27 although it

is a bit far, however, from the 2.1 and 2.5 kpc given by MFJ77

and MDEW01, respectively. A more reﬁned analysis indicates

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

Fig.9. Histograms of stars for spectral type. Left panels for those

inside the cluster limits and right panels for those stars outside

the cluster (see text for explanation).

this peak contains about 20 stars including three O-type stars at

∼ 1.5 kpc. and six other hot stars with distances between 0.9 and

1.6 kpc. The rest of the stars of photometric B-types (eight of

them have evolved MK types) spread over distances from 1 to

2 kpc. Three of them with conﬁrmed evolved MK types are in

a distance range between 1.1 and 1.7 kpc. The M0Ia supergiant

star is placed at 1.8 kpc in our distance estimation but since col-

ors of extremely red stars may be quite uncertain it is likely lo-

cated at the same distance than the others. The WC9 star, placed

at 2.2 kpc, is unlike to be related with this star group. The mean

distance of all the stars in this stellar grouping (allowing for dis-

tance errors) is 1.7 ± 0.4 kpc. This is almost 1 kpc below the

value derived by MDEW01. Compared to other distances com-

puted for Trumpler 27, it is 0.5 kpc less than the one estimated

by MFJ77, 2.1 kpc, and about 0.5 kpc larger than another value

given by The & Stokes (1970), 1.1 kpc, but is in good agreement

with the distance published by Bakker & The (1983), 1.6 kpc

from the Sun. However, the simple grouping by itself -with huge

distance spread- does not indicate the presence of a true open

cluster as we shall see later on. If we look outside the region

of the location of Trumpler 27 in Table 3 we ﬁnd only 35 stars

sharing a similar distance range.

Regarding the second star peak at ∼3.5 kpc seen in the di-

rection to Trumpler 27 core, it is quite far from the distance 2.1

and 2.5 kpc given for Trumpler 27 by MFJ77 and MDEW01.

Moreover, it is wider than the ﬁrst one and tends to show some

kind of coincidence with the evident peak of stars outside the

cluster region at the same distance approximately (see right pan-

els in Fig. 9). In this distance range from 3 to 3.5 kpc there are

more than 110 stars outside the potential cluster core including

3 stars of O-types and one B-type supergiant out of 27 stars with

MK types. The WC7/N6 and the G0I stars, both at 2.4 kpc ap-

proximately, are probably not related with this second peak of

stars either.

The histograms ofFig. 9 are conclusiveinthe sense that early

type stars are found at any distance inside and outside the cluster

Fig.10. CMDs of stars ordered by distance. Symbols are: (+)

stars from 0 to 2 kpc, () for 2-3 kpc, (◦) for 3-4 kpc, (△) for 4-5

kpc and (⋆) stars beyond 5 kpc. Upper panel, all stars aboveB5-

spectral types in Trumpler 27 region. Lower panel, idem above

for stars outside.

region. That is why,in our opinion, diﬀerent authorsarriveat dif-

ferent conclusion about members and distances of this putative

open cluster. An additional proof is provided by the observed

CMDs of early type stars found inside and outside the cluster

boundariesshownin Fig. 10. Here stars havebeen identiﬁed with

diﬀerent symbols according to their distances and immediately

arises that any combination of them may produce a believable

CMD. In particular, the stars in the 3-4 kpc range are responsible

for the left envelope and high density of both CMDs while the

most dispersed stars are those belonging to the 2-3 kpc range.

To be remarked is that inside the limits, where it is assumed

Trumpler 27 to exist, the upper part of the CMD is composed

by stars as near as 2 kpc and as far as more than 5 kpc from the

Sun.

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

From the above analysis we conclude then that the evidence

is not conclusive enough to clearly establish the true nature and

location of Trumpler 27. There is no doubt here that we are deal-

ing with a diﬀuse young population of stars distributed all along

the line of sight with some stellar clumps including all types of

bright stars.

5. Infrared information

Infrared data provided by the GLIMPSEII spring’08 (highly re-

liable) catalogue was cross-correlated with our optical catalogue

to get 2MASS JHKs and IRAC 3.6, 4.5 and 8 µm data. In par-

ticular, JHKs data was used to perform an additional search for

Trumpler 27 as can be seen in Fig. 11 (down to J = 16 mag).

Using all stars in the GLIMPSEII catalogue which lie inside the

boundaries of our observed frames, a radial density proﬁle was

constructed as shown in the upper left panel of Fig. 11. The pro-

ﬁle gives the number of stars per square arcmin computed as the

number of stars found in concentric annuli 1 arcmin wide start-

ing at the current adopted cluster coordinates given in §1. Error

bars are Poisson errors (computed as

√

N with N the number

of stars per square arcmin). The panel conﬁrms that the cluster

is only an artifact produced by the evident dust ring around the

center as shown by the pronounced dip at 3 arcmin from the cen-

ter. This is the same shown in Fig. 6, left panel. Apart from the

dip the star density proﬁle is essentially plain, within the Poisson

errors, showing no evidence of a star clustering.

Close to star No 1 and 1b (MDEW01 numbering) there ex-

ists the maser source OH355.1-0.76 measured by Knapp et al.

(1989) in their circumstellar CO emission survey. Unfortunately

they give no further information on this object since and it was

discarded due to the strong contamination caused by galactic CO

emission over a wide range of velocity. We assume that, proba-

bly, the supergiant star No 1 is the optical counterpart of this ob-

ject since that, in turn, is also a suspected variable (Samus et al.

2010). Across the area surveyed in this article we found optical

counterparts of two YSO candidates (Nos 5338 and 5269 in our

numbering, corresponding to 2MASSJ17362441-3331289 and

2MASSJ17362515-3325521 sources, respectively) and another

star (No 1796, our numbering) with huge infrared excess.

Upper right panel in Fig 11 is the overall (J − H), (H −

K) two color diagram. The intrinsic relation from Koornneef

(1983), solid black line, and the 6 mag reddening vectors

(J−H)

(H−K)

= 1. 7, gray dashed lines, for early and late type

stars are indicated. We have superimposed stars earlier than B5

inside 600 pxls radius (white squares) and outside these limits

(red squares). Despite the fact that data scatter is strong and typ-

ical infrared errors are large, this diagram conﬁrms that stars in

the region of Trumpler 27 are aﬀected by strong variable redden-

ing and shows no indication of a clear cluster sequence. Indeed,

the spread of the data inside 600 pxl is similar to the one shown

by the other early stars outside this limit. Large blue squares and

the arrow show the position of the YSO candidates above men-

tioned and the star No 1796. It seems that more objects in the

zone exhibit large infrared excesses. The other two lower pan-

els are the K, (J − K) and V, (J − K) color-magnitude diagrams.

These two diagrams show the same pattern found in Fig. 4 in the

sense that no clear cluster sequence emerges from them and no

evidence of a pre-main sequence is revealed. The illusory verti-

cal strips seen in the K, (J − K) are composed by stars at very

diﬀerent distances inside the 600 pxl radius and also outside that

limit. Actually, with no other information available to impose

boundary conditions (as the one provided by the CCD in Fig.

4 and MK types) it would be possible to confuse the vertical

Fig.11. The radial density proﬁle in the region of Trumpler 27

-upper left panel. Bars are Poisson errors. The (J − H), (H −

K) color-color diagram of all stars in the region is shown in the

upper right panel. White and red squares are stars in Tables 2

and 3 respectively. The cross indicates the average color errors.

Large blue squares give the position of the YSO candidates and

the arrow points out the position of the star No 1796 outside

the diagram limits. Solid black line is the intrinsic relation from

Koornneef (1983); the two grey dashed lines are color excesses

relations for A

= 6 mag for early and late type stars. The lower

left and right panels show the K, (J − K) and V, (J − K) color-

magnitude diagrams. Symbols as in the upper right panel.

strip with the upper sequence of an unreal cluster. For the sake

of completeness we have also plotted 3.6, 4.5 and 8 µ data in

the adequate diagrams (not shown to save space) and, again, no

evidence for a cluster has been found in any of them.

6. The region toward l = 355

◦

and the inner spiral

structure of the Galaxy

The presence of over 600 stars of very early spectral types in a

small sky area of a quarter of square degree located in a contin-

uous range of distance up to 7 kpc at least from the Sun is an ex-

citing result of our survey that connects this young diﬀuse pop-

ulation to the inner Galactic structure. In the traditional model

of four arms for the galaxy (Russeil 2003, Vall´ee 2005) mov-

ing from the Sun location inwards -in the region of Trumpler

27- there should be Sagittarius-Carina arm, Scutum-Crux arm,

Norma and the Near 3 kpc-arms at 0.6, 3, 4 and 5 kpc re-

spectively. In the proposed recent view face-on of our galaxy

(Churchwell et al. 2009), the cross with Sagittarius-Carina arm

should happen at 1.6 kpc. In particular, this arm looks weak and

narrow; this is, the arm is tiny and not very prominent. In the

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

same sketch the Scutum-Crux arm is located at 3.2 kpc and be-

comes the most prominent feature of the inner galactic structure.

The Norma arm is another tiny structure intersected at 4.4 kpc

and, ﬁnally, the Near 3 kpc arm at over 5 kpc is the less pro-

nounced of all the features. A recent map of the Milky Way has

been presented by L´epine et al. (2010) where the traditional four

arm structure is abandoned and replaced by polygonal arms de-

scribed by molecular CS associated with IRAS sources. Given

the complex structure proposed by L´epine et al. (2010) we pre-

fer to compare our ﬁndings with the traditional description (e.g.

Russeil 2003).

In terms of the stellar components, mainly O-B3-type stars

and open clusters associated with the grand design spiral struc-

ture, the information is scarce as shown by Russeil (2003). The

OB type stars catalogue recently compiled by Reed (2003) has

been used by Carraro (2011) to highlight the spiral structure

around the Sun. Fig.1 in Carraro (2011) demonstrates that only

O-type stars show some sort of clear coincidence with spiral

arms of the Galaxy while B-type stars, on the contrary, show a

smooth distribution up to the reach of the survey made by Reed

(2003). Carraro (2011) also emphasizes the lack of coincidence

of the star sample (O- plus B-types) with the trace of Scutum-

Crux. In the direction we are dealing with, the Reed (2003) cat-

alogue scarcely reaches 5 kpc from the Sun while the number

of conﬁrmed(spectroscopically)O-type stars byMa´ız-Apell´aniz

et al. (2004) -complete only to V = 8 mag- indicates that very

few conﬁrmed stars of this type are found in the direction where

Trumpler 27 is located.

Returning to our data sample we have included stars of

Tables 2 and 3 in the distance vs E

(B−V)

plot of Fig. 12 up to

18 kpc from the Sun. In the ﬁgure we also have marked the dis-

tances to which the spiral arms of the Galaxy should be located

approximately according to the current accepted model of four

arms and their respective widths. We brieﬂy recall that while

arms separation is of the order of 3 kpc the width of them is

quite uncertain. We set arms widths in Fig. 12 using the value of

about 1 kpc given by Vall´ee (2005). Accepting that Sagittarius-

Carina is at less than 1 kpc in the 355

◦

direction we found very

few stars associated with this spiral arm, mostly O and B3-B4,

while the bulk of young stars tend to gather at the distance where

Scutum-Crux and Norma arms are set. The plots in Fig. 12 are

in line with Fig. 1 shown by Carraro (2011) in the sense that

very young stars do not deﬁne conclusively the trace of inner

spiral arms but are distributed evenly across several kiloparsecs

toward the center of the galaxy. This is a singular diﬀerence with

radio observations. In fact, maps of the HI and CO contributions

suggest that these arms are discrete and well outlined structures,

Scutum-Crux being one of the major of them moving toward the

inner galaxy.

The recent study by Dame & Thaddeus (2008) collected spe-

ciﬁc and irrefutable evidence of the presence of the distant coun-

terpart of the Near 3 kpc arm based in CO and previous obser-

vations in HI. The authors estimated that distances from the Sun

for both arms are 5.1 and 11.8 kpc for the Near 3kpc arm and

the Far 3 kpc arm, respectively. It is worth mentioning that stars

at all spectral types (also one O-type) can be found in or close

to the Near 3 kpc arm in Fig. 12. If the tendency in our sample

is correct, for the ﬁrst time we are looking at components of a

very recent star formation process there. From the point of view

of our ﬁndings the relation between the young population and

the Near 3 kpc arm is not controversial. As Dame & Thaddeus

(2008)show, the Near 3 kpc arm extendsmostly belowthe galac-

tic plane at the location of Trumpler 27 and therefore, part of the

young stars in our sample may be spatially connected with this

arm. There are stars even farther in this region (at more than 8

kpc) that have no chance to be related with the Far 3 kpc arm

since it extends mainly above the galactic plane.

From our study, based on an extended sample of early type

stars detected photometrically,we ﬁnd it hard to establish the re-

lation of the inner spiral arms with our stellar population. While

the gas component is expected to trace the path of every inner

arm by means of discrete structures of molecular CO and HI,

stellar distances of young stars do not. Fig. 12 allows us to as-

sure that the trend of the young diﬀuse population extends all

along 7 kpc towards the center of the Galaxy but by no means

can we specify the position and distance from the Sun of each

arm. This diﬃculty arises from a combination of distance un-

certainties with geometrical circumstances. In fact, despite the

process of star formation is active and vigorous it is hard to re-

late it to speciﬁc patterns of spiral structure because of a) the

tight packing of four arms in such a short distance range and b)

our line of sight on the plane is almost at right angles with the

spiral structure.

All the circumstances mentioned above are reﬂected in the

radio observation domain. Very recently, Green et al. (2011)

have been studying the distribution of methanol masers in the

inner galaxy covering the longitude range from -28

◦

to +28

◦

longitude. In particular, in the direction of our analysis, they

found an enhancement of methanol sources which are com-

prehensible only as a superposition of contributions from large

galactic structures confused by intervening spiral arms along the

line of sight. This diﬃculty is exactly the same we found in this

paper.

7. Conclusions

This is the ﬁrst time that an UBVI CCD photometric study is

carried out in the region of the open cluster Trumpler 27 towards

l = 355

◦

. In order to determine the properties of the stellar pop-

ulation, we have applied well known broad-band photometric

techniques. Our goal has been twofold: on one side to recognize

the main parameters of the cluster -if it exists- since several

observers in the past arrived at diﬀerent conclusions and, on

the other side, to analyze the ﬁeld star population present in the

region, since the same studies underlined the presence of a large

number of bright stars that have no relation withthe cluster itself.

An open cluster is, in essence, a decreasingsequence of grav-

itationally bound stellar masses well conﬁned in a volume of

space. In the case of nearby open clusters, the analysis of proper

motions and radial velocities becomes a powerful tool to asses

the cluster membership on any star found in the ﬁeld. As for dis-

tant clusters, the way to assess if a star belongs to it is by resting

on a spectrophotometric study focused on some sort of stellar

overdensity detected against the sky background. In the zone of

Trumpler 27 there is neither reliable proper motions -given the

distances of the stars in the region- nor appreciable star over-

density even using infrared data as shown in Fig. 11, upper left

panel.

From a spectrophotometric point of view we cannot conﬁrm

or rule out deﬁnitely the existence of the cluster since results of

our analysis based on separating early type stars from ﬁeld stars

are not conclusive at all. This is, CMDs showing the same pat-

tern can be constructed with stars at diﬀerent distances from the

Sun neighborhood all the way to the Galactic center. Massive

stars of O-type, evolved B-types, M- and G- super-giants plus

two WR stars are found at all distances and widely separated -

angularly- from each other making it unlikely to connect them,

Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk

Fig.12. The path of reddening with distance according to the star spectral type. The approximate position and width of the inner

spiral arms is shown by dashed columns; the probable beginning of the Bulge is indicated by the arrow. Numbers in parentheses as

in Fig. 8.

from an evolutionary point of view. The diﬀerent CMDs in opti-

cal and infrared are typical of very young star groups superposed

along the line of sight resembling, e.g., OB associations. This

could be the reason for the age spread detected by MDEW01

since the entire star group associated historically with Trumpler

27 is composed by stars which did not form in a same event and

do not share the same spatial location.

Basing on our data, it is diﬃcult to assess the existence of

Trumpler 27 at the distance, with the members and with the

age derived in previous articles already cited. In addition, we

were unable to ﬁnd in our diagrams a star sequence composed

by faint stars that could be related beyond doubt with potential

massive members of Trumpler 27. This is a serious obstacle

and we ﬁnd it unlikely that photometry alone could solve the

question in the future. Certainly, the controversy may be settled

by means of an exhaustive spectroscopic study able to reach

also faint stars. An interesting point to interpret, in terms of

star formation too, is the lack of any evident HII region in the

zone as expected in a region of massive star formation. WR

and O-Type stars are usually injecting a tremendous amount of

ultraviolet photons in the surrounding gas which should be seen

in emission but there are no clear traces of emitting gas.

As far as the diﬀuse young star population is concerned, in the

direction l = 355

◦

, our star sample shows an even distribution.

This suggests that early type stars deﬁne a continuous superpo-

sition of stars with no density peaks from one arm to the next,

except for the prominent clump at the location of the Scutum-

Crux arm, as seen in Fig. 9.

The ﬁndings of this paper can be connected with the ones

from Carraro (2011) at l = 314

◦

. Firm evidence was found there

of a young stellar population associated with Carina-Sagittarius

and Scutum-Crux arms. Moreover, the existence of an even star

distribution in the direction l = 314

◦

, where arms are more open

than in the Galactic center direction, was found. This conﬁrmed

the outcome of old (e.g. Muzzio and Levato 1980 and references

therein) and more recent (Carraro & Costa 2009, Baume et al.

2009) studies that provided robust optical evidences of their ex-

istence and their potential as spiral arm indicators.

However, in the present study which looks towards the cen-

ter of the Galaxy, four spiral arms are closely packed in only 5

kpc from the Sun. Therefore, the task of disentangling one struc-

ture from the next is more cumbersome, and would require the

support of an improved radio trigonometric parallax method to

measure accurate distances and proper motions of obscured star

forming regions in the Milky Way (Brunthaler et al. 2011).

Meanwhile, we believe it is of extreme value to continue

our program which aims at studying the inner disk stellar pop-

ulations using UBV and infrared photometry by extending it to

other absorption windows, as in the case of Trumpler 27. Young

stellar populations can be very well tracked using UBV photom-

etry and when combined with ﬁndings from radio observations,

a better picture of large structures can be achieved certainly.

Acknowledgements. G. Carraro expresses his gratitude to the Las Campanas

Observatory staﬀ, and in particular to Patricio Pinto for the excellent support

during various observing runs. We also thank Sandy Strunk for reading carefully

the manuscript. G. Perren and R.A. V´azquez acknowledge the ﬁnancial support

from the CONICET PIP1359. This research has made use of the NASA/IPAC

Infrared Science Archive, which is operated by the Jet Propulsion Laboratory,

California Institute of Technology, under contract with the National Aeronautics

and Space Administration.

We acknowledge useful and constructive comments from an anonymous ref-

eree which allowed us to improve the manuscript.

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